Hydrostatic units are a common way of achieving a stepless transmission for use in an automobile. Stepless transmissions are desirable because they are steplessly adjustable over a wide range, and can provide improved efficiency in packaging and fuel economy. A common way to achieve stepless gearing is through the use of a hydrostatic unit. Hydrostatic units consist of a pump, with a variable displacement volume, and a motor, which is driven by the pump, having either a fixed or variable displacement volume. A power source coupled to the transmission provides power to a planetary gearset, which in turn splits the power between the hydrostatic unit and the output.
One problem that exists in current continuously variable transmissions that utilize hydrostatic units is that there is no “mechanical neutral,” meaning that when a neutral condition is desired, there is no complete disconnect from the power source to the transmission. In most current designs, neutral is achieved by adjusting the swashplate of the pump in the hydrostatic unit such that no power is transferred to the motor. One major drawback to this design is that residual drag torque in the transmission can still transfer a small amount of torque to the output, thereby not allowing for a true neutral position; also, any deviation in the tolerance of the displacement volumes in the hydrostatic unit may also cause a small amount of torque to be transferred through the transmission, which would also negate the vehicle being in neutral.
Another problem with current designs using a hydrostatic unit involves trying to achieve a reverse gear. This is accomplished by reversing the operation of the hydrostatic unit. In the reverse configuration, the motor acts as the pump, and the pump acts as the motor. The drawback in this method of achieving reverse comes from the displacement volumes of the pump and motor in the hydrostatic unit being configured to provide the proper amount of torque when the vehicle is going in a forward direction. In reverse, a very small amount of torque is transferred through the hydrostatic unit to drive the gearset, resulting in the vehicle moving very slow, and with little power. Also, when in reverse, because of the displacement volumes in the hydrostatic unit, the engine may run at very high rpm to achieve reverse using this method, making it very inefficient.